Evolution of Zygotic Linkage Disequilibrium in a Finite Local Population

One crucial feature of zygotic linkage disequilibrium (LD) analysis is its direct use of diploid genotyping data, irrespective of the type of mating system. Previous theories from an evolutionary perspective mainly focus on gametic LD, but the equivalent development for zygotic LD is not available. Here I study the evolution of zygotic LD and the covariances between gametic and zygotic LDs or between distinct zygotic LDs in a finite local population under constant immigration from a continent population. I derive the analytical theory under genetic hitchhiking effects or in a neutral process. Results indicate that zygotic LDs (diploid level) are more informative than gametic LD (haploid level) in indicating the effects of different evolutionary forces. Zygotic LDs may be greater than or comparable to gametic LD under the epistatic selection process, but smaller than gametic LD under the non epistatic selection process. The covariances between gametic and zygotic LDs are strongly affected by the mating system, linkage distance, and genetic drift effects, but weakly affected by seed and pollen flow and natural selection. The covariances between different zygotic LDs are generally robust to the effects of gene flow, selection, and linkage distance, but sensitive to the effects of genetic drift and mating system. Consistent patterns exist for the covariances between the zygotic LDs for the two-locus genotypes with one common genotype at one locus or without any common genotype at each locus. The results highlight that zygotic LDs can be applied to detecting natural population history.     

An additional gametophyte factor in the lima bean

Summary A second gametophyte factor (Ga ₂) is described in lima beans. This gene produces an interaction between the male gametophyte and style such that: (a)Ga ₂ pollen functions irrespective of stylar genotype; (b)ga ₂ pollen is functional onga/ga styles but is unable to compete withGa ₂ pollen onGa ₂/Ga ₂ orGa ₂/ga ₂ styles.Ga ₂/ga ₂ is linked with theP/p locus (recombination fraction =0.244±.017); it is therefore probably located in linkage group I.Some similarities and differences between gametophyte factors and incompatibility systems and some factors which might influence the establishment and maintenance ofga alleles in populations are discussed. Evidence is presented that theGa ₂ allele arose as a recent mutationga ₂Ga ₂ in one of the pure-line parents of the families studied.

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Zusammenfassung An Limabohnen-Familien, die für das GenP/p für Samenschalenfarbe heterozygot sind, wurden persistente Spaltungsabweichungen analysiert.Ein zweiter gametophytischer Faktor (Ga ₂) wird beschrieben. Dieses Gen bewirkt eine Interaktion zwischen dem männlichen Gametophyten und dem Griffel in der Form, daß a)Ga ₂-Pollen unabhängig von dem Genotyp des Griffels funktionsfähig ist. b)ga ₂-Pollen zwarga/ga-Griffel befruchten, aber aufGa ₂/Ga ₂-Griffeln oderGa ₂/ga ₂-Griffeln mitGa ₂-Pollen nicht konkurrieren kann.Ga ₂/ga ₂ ist mit dem LocusP/p gekoppelt (Rekombinationswert 0,244±0,017) und daher wahrscheinlich in Koppelungsgruppe I lokalisiert.Ähnlichkeiten und Unterschiede zwischen gametophytischen Faktoren und Inkompatibilitätssystemen sowie Faktoren, die das Auftreten und die Ausbreitung vonga-Allelen in Populationen beeinflussen können, werden diskutiert. Es wird der Beweis erbracht, daß dasGa ₂-Allel in einer der reinen Elternlinien der untersuchten Familien als eine neue Mutationga ₂Ga ₂ entstanden ist.

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This work was supported in part by a grant from the National Science Foundation G-14991.     

A Theoretical Model for Estimating Linkage in F2 Populations with Distorted Single Gene Segregation

A model is presented which allows estimation of linkage from dihybrid F₂ populations with distorted single gene segregation by applying the maximum-likelihood method. For different selection processes operating on one locus at either the gametic or the zygotic level, it can be demonstrated that, if the deficit is previously taken into account, testing for free recombination can be carried out without prior knowledge of the causes of this deficit. In the presence of linkage, the expected frequencies of two phenotypic classes depend on whether gametic or zygotic selection is operating. The remaining two classes can be utilized for the estimation of linkage as their frequency ratio is independent of these selection types. The application of this procedure to situations with coupling, incomplete penetrance, gametic and zygotic selection is discussed.     

Estimating exotic gene flow into native pine stands: zygotic vs. gametic components

Monitoring contemporary gene flow from widespread exotic plantations is becoming an important problem in forest conservation genetics. In plants, where both seed and pollen disperse, three components of exotic gene flow with potentially unequal consequences should be, but have not been, explicitly distinguished: zygotic, male gametic and female gametic. Building on a previous model for estimating contemporary rates of zygotic and male gametic gene flow among plant populations, we present here an approach that additionally estimates the third (female gametic) gene flow component, based on a combination of uni‐ and biparentally inherited markers. Using this method and a combined set of chloroplast and nuclear microsatellites, we estimate gene flow rates from exotic plantations into two Iberian relict stands of maritime pine (Pinus pinaster) and Scots pine (Pinus sylvestris). Results show neither zygotic nor female gametic gene flow but moderate (6–8%) male gametic introgression for both species, implying significant dispersal of pollen, but not of seeds, from exotic plantations into native stands shortly after introduced trees reached reproductive maturity. Numerical simulation results suggest that the model yields reasonably accurate estimates for our empirical data sets, especially for larger samples. We discuss conservation management implications of observed levels of exposure to nonlocal genes and identify research needs to determine potentially associated hazards. Our approach should be useful for plant ecologists and ecosystem managers interested in the vectors of contemporary genetic connectivity among discrete plant populations.     

Polyploidy with an arbitrary mixture of chromosome- and chromatid segregation

Summary The gametic algebraG is constructed for a random mating population of 2r-ploid individuals which differ in a single locus with the allelesA anda. It is assumed that every kind of segregation between chromosome- and chromatid segregation occurs with a given probability. This algebraG is a convex combination of 2r+1 genetic algebras which have a common canonical basis. The train roots of these algebras are calculated and shown to be monotonically descending. The algebraG possesses a one-dimensional manifold of idempotents. With a generalization of Gonshor's theorem on the convergence of the sequence of plenary powers of an element of unit weight it is shown that for every initial gametic distribution the distribution in the following generations converges towards an equilibrium state whose coordinates are polynomials in the frequency of the alleleA in the initial generation.     

Certationsversuch zur Gonenkonkurrenz unter dem Einfluß des Locus Ga bei Oenothera

Zusammenfassung Mit dem Pollen von zwei Oenothera-Bastarden, die für die Testgene s, de und ein gametophytisch im Pollen wirksames Gen ga heterozygot waren, wurden Certationsversuche in Rückkreuzungen auf Oe. hookeri sulfurea durchgeführt. Weder eine Veränderung der Bestäubungsdichte noch die Unterbrechung des Pollenschlauchwachstums durch Abschneiden der Griffel während der Wachstumsphase hat einen Einfluß auf die Spaltungsverhältnisse der Testmerkmale in der Nachkommenschaft. Es ist demnach äußerst unwahrscheinlich, daß die Konkurrenz durch Unterschiede der Wachstumsgeschwindigkeit der Pollenschläuche im Griffel entschieden wird. Die Benachteiligung der Gametophyten mit dem Allel ga ^– muß entweder kurz vor der Befruchtung im Fruchtknoten liegen oder sich in den ersten Stadien der Pollenschlauchentwicklung bei der Keimung und dem Wachstum in der Narbe bemerkbar machen. Aus den Versuchsdaten ergibt sich eine erhebliche Variabilität des crossing-over im untersuchten Chromosomenbereich.

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Certation experiment concerning competition between pollentubes under the influence of the locus Ga in Oenothera

Summary The material for the experiments were two Oenothera-Hybrids with the complexes ^h hookeri and flavens which were heterozygous for the gametophytic gene ga and for the genes s and de. Certation experiments were done by pollinating the styles of Oe. hookeri sulfurea with different amounts of pollen grains and by interruption of pollen tube growth by cutting off the styles at different times after pollination.There is no correlation between the experimental treatment and the segregation for the two genes in the progeny. The competition between pollen cells with the alleles respectively ga ⁺ and ga ^– therefore cannot be due to differences in growth rates during the developmental stage of pollen tube-growth in the style but must occur during the germination of the pollen on the stigma or in the last stage of development of the pollen tubes shortly before fertilization. The data reveal great variability of crossing-over in the chromosome-region under investigation.

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Angenommen durch G. Melchers und W. Seyffert     

Gametic non-random associations in North-West African populations ofDrosophila melanogaster

Three North-west African populations ofDrosophila melanogaster were analyzed for chromosomal and enzyme polymorphism as well as for gametic associations between non allelic elements mapped on the same chromosome arm. Strong geographic heterogeneity was found between populations, both for enzyme and chromosomal polymorphism. Out of the 43 possible comparisons for gametic association, 14 (32.5%) were statistically significant.     

Monophyletic origin of naked barley inferred from molecular analyses of a marker closely linked to the naked caryopsis gene (<Emphasis Type="Italic">nud</Emphasis>)

To elucidate the origin of naked barley, molecular variation of the marker sKT7 tightly linked to the nud locus was examined. A total of 259 (53 wild, 106 hulled domesticated, and 100 naked domesticated) barley accessions were studied. Restriction analysis of the sKT7 PCR-amplified product revealed the alleles I, II, III, and IV. All four alleles were found in wild barley, but allele IV was found only in a single accession from southwestern Iran. Hulled domesticated accessions showed alleles I, II, or III, but all naked domesticated accessions had allele IV. The distribution of allele IV in wild barley and its pervasive presence in naked domesticated lines support the conclusion that naked barley has a monophyletic origin, probably in southwestern Iran. The available results suggest two scenarios for the origin of naked barley: either directly from a wild barley with allele IV or from a hulled domesticated line with allele IV that later became extinct. Naked domesticated accessions from different regions of the world have extremely homogeneous DNA sequences at the sKT7 locus, supporting the monophyletic origin of naked barley. For allele IV, four haplotypes (IVb to IVe) were found in 30 naked accessions: IVb was predominant (66.7%) and widely distributed, while the other three haplotypes, differing by only one nucleotide at different positions relative to IVb, showed a localized distribution. The geographical distribution of the haplotypes of sKT7 allele IV suggests migration routes of naked domesticated barley in central and eastern Asia.Communicated by F. Salamini     

Gonenkonkurrenz bei Oenothera unter dem Einflu\ eines gametophytisch wirksamen Gens in der ersten Koppelungsgruppe sowie ein Modell für die Untersuchung verzweigter Koppelungsgruppen

Zusammenfassung Die Nachkommenschaften aus Rückkreuzungen des Pollens von Bastarden mit den durch zwei Test-gene und den Translokationspunkt markierten Komplexen ^h hookeri und flavens zeigen Spaltungszahlen, die gesichert von der Mendel-Erwartung abweichen.Es wird ein gametophytisch wirksamer Genlocus in der ersten Koppelungsgruppe angenommen, dessen Allele ga ⁺ und ga ^- so auf die Entwicklung der Gametophyten (Pollen und Pollenschläuche) einwirken, daß den Gameten mit verschiedenen ga-Allelen eine unterschiedliche Befruchtungswahrscheinlichkeit zukommt, wenn sich die Pollenkörner auf einer heterozygoten Mutterpflanze entwickelt haben.Es werden in mehreren Stufen genetische Modelle entwickelt, bei denen die den Allelen ga ⁺ und ga ^- zugeordneten Befruchtungswahrscheinlichkeiten, die Austauschwahrscheinlichkeiten auf den untersuchten Genstrecken und die Verzweigung der Koppelungsgruppe durch einen Translokationspunkt berücksichtigt werden.Anhand dieser Modelle werden Voraussagen über die zu erwartenden Relationen zwischen den relativen Häufigkeiten der einzelnen Gametenklassen unter verschiedenen Voraussetzungen über die Anordnung der Genloci in der Koppelungsgruppe möglich.Durch die Formulierung des Modells kann es über den vorliegenden Fall hinaus allgemein sowohl für die Lokalisation von gametophytisch wirksamen Loci als auch für die Lokalisation beliebiger Genloci in verzweigten Koppelungsgruppen verwendet werden.Es erweist sich als unmöglich, aus Rückkreuzungsdaten den Grad der Benachteiligung von ga ^- gegenüber ga ⁺ zu bestimmen. Aus empirischen Daten kann vielmehr nur ein Schätzwert gewonnen werden, in den die Befruchtungswahrscheinlichkeiten und der Austausch auf der Strecke zwischen Ga und dem Testlocus eingehen.Durch Vergleich der empirischen Daten mit den Modellen wird versucht, die Anordnung der Testloci und des Locus Ga in bezug auf den Translokationspunkt zu bestimmen. Die meisten Befunde sprechen für eine verzweigte Koppelungsgruppe, in der die drei Loci s, de und Ga auf drei Armen der Translokationsfigur angeordnet sind. Eine lineare Anordnung in zwei Chromosomenarmen kann jedoch nicht mit Sicherheit ausgeschlossen werden.Die Austauschwerte für die untersuchten Genstrekken zeigen eine erhebliche Variabilität zwischen Bastarden und zwischen verschiedenen Nachkommenschaften eines Bastards.

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Competition between the male gametophytes in Oenothera under the influence of a gametophytic gene in the first linkage group and a model for investigations of branched genetic maps

Summary The progeny ratios obtained in male backcrosses of Oenothera hybrids to the complexes hookeri and flavens (identified by two marker genes and by the translocation break) showed statistically significant differences from Mendelian expectations.Presence of a gametophytic gene in the first linkage group is proposed. Its alleles ga ⁺ and ga ^- are thought to control development of the male gametophytes in such a manner that pollen grains carrying the different ga alleles and developing in a heterozygous female plant, will fertilize the female gamete with different average frequencies.A series of genetic models is developed, all taking into account the relative probability of the alleles ga ⁺ and ga ^- to function in fertilization, the probability of crossing over between the gene loci studied, and the branched configuration of the linkage group, due to the presence of the translocation.Based on these models, the relationships between the relative frequencies of the different classes of gametes are predicted for several theoretically possible arrangements of the genes within the linkage group.The models have general application for the localization of gametophytic genes and for the localization of genes in branched linkage groups.It turns out to be impossible to give an estimate for the disadvantage of ga ^- as compared to ga ⁺ from the backcross data alone. The empirical data permit only a combined estimate of the probabilities of fertilization by ga ^- and of crossing over between Ga and the locus of the marker gene.Comparison between the empirical data and the model makes it possible to determine the location of the marker loci and the locus of Ga with respect to one another. A branched linkage group appears most likely, but a linear arrangement in two chromosome arms cannot be ruled out.The observed cross-over values, and consequently, the map distances between the genes studied, are highly variable, both in different hybrids and in the progeny of a single hybrid.

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Angenommen durch G. Melchers und W. Seyffert     

Selection component analysis of natural polymorphisms using population samples including mother-offspring combinations, II

Population samples including mother-offspring combinations provide information on the selection components: zygotic selection, sexual selection, gametic seletion and fecundity selection, on the mating pattern, and on the deviation from linkage equilibrium among the loci studied. The theory for the analysis of a sample consisting of adult individuals including combinations of a mother and one randomly chosen offspring in the case of n polymorphic autosomal loci with m_a alleles at the ath locus, a = 1, 2,…, n, is developed for an organism with discrete non-overlapping generations. In this general analysis complete determination of the genotypes is assumed. The modifications in the analysis for the case n = 2 and m₁ = m₂ = 2 when the two double heterozygotes cannot be separated are discussed, and the analysis is illustrated by data from a population of Zoarces viviparus (a marine teleost).     

Comparison of different techniques for gene transfer into mature and immature tobacco pollen

The particle gun, cocultivation withAgrobacterium tumefaciens, and imbibition in DNA solutions were compared as methods to transfer DNA into mature and immature pollen ofNicotiana tabacum. Bombardment of mature pollen with the β-glucuronidase gene cloned behind the pollen-specific PA2 promoter of the chalcone isomerase gene ofPetunia hybrida resulted in the expression of the β-glucuronidase gene in 0.025% of the pollen grains. Bombardment of younger stages followed byin vitro maturation also resulted in the formation of mature pollen that expressed β-glucuronidase, although at a lower frequency. Cocultivation of pollen duringin vitro maturation orin vitro germination withAgrobacterium tumefaciens did not yeild β-glucuronidase-expressing pollen. In these cases, an intron-containing β-glucuronidase gene was used which effectively prevented β-glucuronidase expression in the bacteria. Imbibition of mature, dry pollen in various DNA solutions of the same constructs also did not lead to the formation of β-glucuronidase expressing pollen.     

Influence of asymmetrical mating patterns and male reproductive success on the maintenance of sexual polymorphism in Acer pictum subsp. mono (Aceraceae)

Populations of Acer species often contain more than three sex phenotypes with complex sexual polymorphism including duodichogamy, protandry and protogyny. We identified the mechanisms that maintain sexual polymorphism in Acer pictum subsp. mono, a temperate tree from northern China, by investigating maternal mating patterns and male reproductive success. We used paternity analyses to estimate rates of outcrossing and disassortative mating, as well as male outcrossed siring success, in a population of A. pictum subsp. mono with uneven sex phenotype ratios (duodichogamous 69.1%, protandrous 19.6%, protogynous 11.3%). We used a pollen‐transfer model to investigate whether the unequal ratios of sex phenotypes could be explained by the observed patterns of mating. Most progeny resulted from outcrossing, particularly disassortative among the sex phenotypes. Although the duodichogamous phenotype showed a significant amount of intraphenotypic mating, the frequency did not exceed that of disassortative mating. We detected no significant differences in male outcrossed siring success among the sex phenotypes. The pollen‐transfer model demonstrated that sex phenotype ratios could be maintained by the observed mating pattern in the population. Our results indicate that disassortative mating among the sex phenotypes can maintain sexual polymorphism in A. pictum subsp. mono and that ratios biased towards duodichogamy can result from frequent intraphenotypic mating in this phenotype.     

Development of polymorphic microsatellite and single nucleotide polymorphism markers for Cercospora beticola (Mycosphaerellaceae)

The plant pathogenic fungus, Cercospora beticola, causes the most important foliage disease of sugar beet. A previous study has shown that isolates of opposite mating types are present in equal proportions in natural populations; therefore, the aim of this study was to develop highly reproducible polymorphic markers for analysing populations of C. beticola. Five microsatellite and four single nucleotide polymorphism (SNP) markers were developed that allow rapid screening of genetic diversity in C. beticola. Six populations were screened with these markers and all were found to be in gametic equilibrium, indicating random mating in C. beticola.     

Direct and indirect responses to selection on pollen size in Brassica rapa L.

Pollen size varies little within angiosperm species, but differs extensively between species, suggesting the action of strong selection. Nevertheless, the potential for genetic responses of pollen size to selection, as determined by additive genetic variance and genetic correlations with other floral traits, has received little attention. To assess this potential, we subjected Brassica rapa to artificial selection for large and small pollen during three generations. This selection caused significant divergence in pollen diameter, with additive genetic effects accounting for over 30% of the observed phenotypic variation in pollen size. Such heritable genetic variation suggests that natural selection could effect evolutionary change in this trait. Selection on pollen size also elicited correlated responses in pollen number (–), flower size (+), style length (+), and ovule number (+), suggesting that pollen size cannot evolve independently. The correlated responses of pollen number, flower size and ovule number probably reflect the genetically determined and physically constrained pattern of resource allocation in B. rapa. In contrast, the positive correlation between pollen size and style length may represent a widespread gametic‐phase disequilibrium in angiosperms that arises from nonrandom fertilization success of large pollen in pistils with long styles.     

The R1 gene conferring race-specific resistance to Phytophthora infestans in potato is located on potato chromosome V.

Summary Late blight in potato is caused by the fungusPhytophthora infestans and can inflict severe damage on the potato crop. Resistance toP. infestans is either based on major dominantR genes conferring vertical, race-specific resistance or on minor genes inducing horizontal, unspecific resistance. A dihaploid potato line was identified which carried theR1 gene, conferring vertical resistance to allP. infestans races, with the exception of those homozygous for the recessive virulence allele of the locusV1. The F₁ progeny of a cross between this resistant parent P(R1) and P(r), a line susceptible to all races, was analysed for segregation ofR1 and of restriction fragment length polymorphism (RFLP) markers distributed on the potato RFLP map comprising more than 300 loci. TheR1 locus was mapped to chromosome V in the interval between RFLP markers GP21 and GP179. The map position ofR1 was found to be very similar to the one ofRx2, a dominant locus inducing extreme resistance to potato virus X.     

Quantification of the zygotic barrier between interbreeding taxa using gene flow data

Hybridization and introgression via interspecific gene flow are common processes in the plant kingdom. The effectiveness of these processes is governed by the strengths of multiple zygotic barriers. These barriers have often been quantified in artificial settings using laborious and time‐consuming hand‐pollination experiments, but their quantification is nonexistent in the landscape. In this study, we utilized gene flow data within a spatially explicit simulation to assess the strengths of zygotic barriers. Our model system consisted of Populus nigra and its hybrid, P. × canadensis, which interbreed under natural conditions. The study population was located in the floodplain of the Eder River in central Germany. Pollen‐mediated introgression rates from hybrid males into the seeds of individual female trees were used as the target pattern using an inverse modeling approach. Simulations that treated pollen from both taxa equally revealed a large discrepancy between the observed and modeled rates of introgression for both taxa. The discrepancy was reduced by introducing a zygotic barrier against the pollen from the hybrid males. The best model outcome values indicated comparably strong zygotic barriers acting against pollen‐mediated introgressive gene flow into the two parental taxa, P. nigra and P. × canadensis. The sensitivity of our model was tested by applying different dispersal functions. Four common probability density functions were used along with a pollen dispersal function that had previously been fitted to gene flow data from the same dataset. The best barrier value was almost independent of the dispersal functions used here. Moreover, it was within the range previously determined in hand‐pollination‐based investigations, validating our model. These data indicate that the inverse modeling approach is a powerful method for quantifying hidden processes, and we discuss its use as a valuable tool for generating new insights into plant mating systems that are relevant to evolutionary biology and risk analyses in conservation efforts.     

The evolution of intrinsic reproductive isolation in the genus Cakile (Brassicaceae)

In theory, adaptive divergence can increase intrinsic post‐zygotic reproductive isolation (RI), either directly via selection on loci associated with RI, or indirectly via linkage of incompatibility loci with loci under selection. To test this hypothesis, we measured RI at five intrinsic post‐zygotic reproductive barriers between 18 taxa from the genera Cakile and Erucaria (Brassicaceae). Using a comparative framework, we tested whether the magnitude of RI was associated with genetic distance, geographic distance, ecological divergence and parental mating system. Early stages of post‐zygotic RI related to F₁ viability (i.e. initial seed set) tended to be stronger than later stages related to F₁ fecundity (i.e. flower number, fruit number). Mating system significantly influenced early stages of RI, such that RI was lowest when the mother was selfing and father was outcrossing, consistent with an imbalance between sink strength and resistance to provisioning. We found little evidence that adaptive divergence accelerates the evolution of intrinsic post‐zygotic RI, consistent with a nonecological model of evolution that predicts the nonlinear accumulation of RI and RI asymmetry with time (i.e. genetic distance), irrespective of adaptive divergence. Thus, although certain aspects of ecological divergence do not appear to have contributed strongly to the evolution of RI in this system, divergence in mating system actually reduced RI, suggesting that mating system evolution may play a significant role in speciation dynamics.     

Evolution du polymorphisme enzymatique dans des populations expérimentales de Drosophila melanogaster III. Déséquilibre de linkage entre les locus Adh et α-Gpdh

Evolution of enzyme polymorphism in experimental populations of Drosophila melanogaster III. Linkage disequilibrium between alleles of the Adh and -Gpdh loci — The evolution of gametic frequencies and linkage disequilibria between alleles of the Adh and -Gpdh-loci was followed in two experimental populations of Drosophila melanogaster maintained at different temperatures. The results observed were compared with those expected on the basis of theoretical models of gametic selection. Gametic fitness values were estimated from the analysis of the productivity of the different homozygous genotypes.Our experimental results indicate that the selection favours an association between the alleles Adh-F and -Gpdh-S, but it is impossible to generate and/or maintain a stable linkage disequilibrium between the two alleles.     

Monoploids in Zea mays L. following crosses with untreated and X-rayed pollen

Monoploids in Zea mays L. occur spontaneously among individual diploid seedlings. Plants with the gametic chromosome number have also been detected among members of multiple seedlings of maize and numerous other species of angiosperms. Previous reports disclosed that Xirradiation of the pollen successfully stimulated reduced parthenogenesis in some other angiosperms, but the results of X-ray treatment were inconclusive in maize. Therefore, a tester stock of maize homozygous for lg _land gl _l was crossed with pollen from inbred CI3A, carrying the dominant alleles. The pollen was exposed to 0, 1000, 2000, and 4000r units of X rays. chromosome counts were made from root tips of plants exhibiting both recessive phenotypes to establish the frequencies of monoploids in the control and X₁ populations.Monoploids were more abundant among the individual seedlings from crosses with untreated pollen than in the X₁ populations. X irradiation of the pollen is not a feasible method for the induction of monoploids in maize. The X-ray treatments greatly increased the frequency of multiple seedlings, and deficiencies were numerous among them. The members of a set of multiple seedlings were always genetically identical, and no monoploid members occurred. It is concluded that the induced deficiencies caused atypical development resulting in zygotic or embryonic cleavage.Scientific Article No. A2070, Contribution No. 5023 of the Maryland Agricultural Experiment Station, Department of Botany.     

PATTERNS OF MATING IN WILD SUNFLOWER HYBRID ZONES

Theory predicts that homoploid hybrid speciation will be facilitated by selfing, yet most well-documented hybrid species are outcrossers. One possible explanation for this puzzle is that conditions in hybrid populations may favor selfing, even in otherwise outcrossing species. For example, in self-incompatible plants, mixtures of self and interspecific pollen often induce selfing. Here, we examine patterns of mating in three hybrid zones and four “pure” populations of Helianthus annuus and H. petiolaris, wild, self-incompatible sunflower species that are thought to have parented three homoploid hybrid species. Fourteen to 16 maternal families from each pure population and 44–46 maternal families from each hybrid zone were analyzed for seven polymorphic isozyme loci. Maximum-likelihood (ML) methods were used to estimate multilocus outcrossing rates (T_m) and hybridization frequencies for each maternal family, each phenotypic group within each hybrid zone (annuus-like, hybrid, and petiolaris-like), and each population. As predicted for self-incompatible species, all four parental populations have outcrossing rate ML estimates of 1.0. Within the hybrid zones, outcrossing rates were lowest in the H. annuus–like fraction of the population (0.73, 0.72, and 0.74 in the three hybrid zones, respectively), largely intermediate in the H. petiolaris–like group (0.94, 0.90, and 0.94), and highest in the hybrid group (0.97, 0.93, and 0.97). Although outcrossing rates are lower in hybrid zones than in pure populations, it is unlikely that the observed decrease facilitates hybrid speciation because outcrossing rates in the critical hybrid fraction of the population do not differ significantly from 1.0. Dividing the outcrossed pollen pool into intraspecific and interspecific components revealed that maternal plants are largely fertilized by conspecific pollen, confirming an important role for pollen competition as a reproductive barrier. Highly sterile hybrid plants do not appear to discriminate between parental species pollen, but hybrids with higher fertility tend to be fertilized by pollen from the parental group they resemble genetically. Thus, gametic selection leads to substantial assortative mating in these hybrid zones.